It is necessary to mention, however, that the principle itself of the experiment has been objected to, on the ground that the gradation of temperature must always be much more rapid in such an experiment than in the actual case of the Atlantic Ocean. This objection, however, is, in reality, based on a misapprehension. It is sufficient that the difference of temperature at the two ends of the trough should correspond to the difference between the temperature of the arctic and equatorial seas; and it is a matter of no importance whatever that the real rate of gradation should be represented. The case may be compared to the illustration of the descent of water to form springs or the like. Here an experiment would be valid in which the outflow of the illustrative spring was obtained by causing the vent to be so much below the level of the reservoir, though the slope from the reservoir to the vent were very much greater than in the case of any-natural spring. Just as in the case of a spring it is the difference of level, and not the rate of slope, which is effective in causing the rate of outflow, so in the case of the oceanic vertical circulation, it is the actual difference of temperature, and not the rate of gradation, which produces the activity of the circulation.

Another objection has been urged against the 'heat and cold theory' by a very skilful mathematician, Mr. Croll. He reasons on this wise: Since the water which is carried from the equator to the latitude of England1 (say) must have partaken, when at the equator, of the earth's rotation there, which has a velocity of more than 1,000 miles per hour eastwards, whereas, when it reaches our latitudes, it partakes of a rotation-movement reduced to about 620 miles per hour, it follows that, neglecting the drift motions as relatively quite insignificant, friction has deprived the water which has thus travelled from the equator to our latitudes of a velocity amounting to no less than 380 miles per hour. If friction is thus effective, how utterly inconceivable is it, says Mr. Croll, that the descending currents of Dr. Carpenter's theory (or the ascending currents of the evaporation theory) should maintain their motion. Hence, Mr. Croll is an earnest advocate of the trade-wind theory.

The worst of this reasoning is that it proves too much. If friction is so effective, then when the trade-winds flag, as we have seen that they do, the ocean currents ought to be brought to a standstill. Moreover, the submarine currents exist, and the wind theory leaves them unexplained. The fact really is that Mr. Croll's reasoning has no application to a system of fluid circulation, where the advance of one part of the fluid is always made room for, so to speak, by the removal of that which it replaces. We might equally well apply Mr. Croll's reasoning to prove that a river cannot flow because of the friction along its banks, as to show that ocean currents cannot flow within their liquid banks. Indeed, many of the points in dispute in this matter of oceanic circulation may be excellently illustrated by considering the case of a river. I propose to draw this paper to a conclusion by setting forth such an illustration. My readers will not fail to recognise the opinions here severally parodied, so to speak, and so to infer the theory which I regard as affording, on the whole, the best explanation of the observed relations.

1 I present the general nature of Mr. Croll's reasoning, without following him in details.

'Shallow persons,' might one say, 'have launched all sorts of stupidities upon the Mississippi River. Physical geographers have deluged the world with their assumptions respecting it; theorists of all kinds have floated their notions upon it. One says that it brings down, past Baton Rouge and New Orleans, the drainage of half the United States; others ascribe to it the detritus around the delta of that great river which flows into the Gulf of Mexico; yet others consider that it breeds the fogs infesting the path of the great stream which flows from Vicksburg to Placquemines.' All this is utter nonsense. The Mississippi has no more to do with the great stream flowing through Louisiana than with the Thames at London. The real Mississippi is a stream of singular purity, and presents other characteristics clearly recognisable as far as its junction with the Missouri; but in the stream which runs past St. Louis none of the characteristics of the Mississippi can be traced. Here, to all intents and purposes, the Mississippi comes to an end. As for the cause of the motion of the great stream itself there can be little question. Some have urged that it is due to the gradual slope of the land; but in all the experimental illustrations of the effects of such slope which we have yet seen, the inclination has been monstrously exaggerated. If slope were the cause of the river's flow, then unquestionably the effective part of the action must reside in the Rocky Mountains, and not in the great reaches of the river. We admit that the chief bulk of the river lies in the great reaches; but this fact has no bearing, we assert, on the question at issue. However, it is demonstrable that no cause of this sort can be in question. For let the following reasoning be carefully marked. In Wisconsin, in 40° north latitude, the river partakes of the earth's rotation motion, there equal in rate to about 800 miles per hour; in Louisiana, in 30° north latitude, the river still partakes of the earth's rotation movement, here equal to about 900 miles per hour. Hence, were it not for the friction exerted by the banks, the water of the river in Louisiana would be flowing at the rate of 100 miles per hour westwards. If, then, friction deprives the river of this enormous velocity-as it obviously does-how much more must it deprive the river of the minute velocity of four or five miles per hour due to slope or inclination. It is certain, therefore, that the flow of the stream is due to the prevalent northerly winds of the so-called Mississippi valley. There are not wanting those, indeed, who assert that this cannot be the case, because northerly winds are not prevalent in this region. But the singular wrong-headedness of this reasoning renders reply unnecessary. That the flow of the great stream is caused by these winds is as certain as the rotundity of the earth.

From English Mechanic for July and August 1872.